Extruding equally along a path
You can try this, it might work for you.


In Edit mode, select the first edge of your road (Left or Right) using alt-click.
Duplicate (Shift-D), hit Escape, then split into a new mesh (P, choose "Selection").
In object mode, select the new object you created (should be a single line of connected vertices), tab into edit mode, and extrude along the Z axis.  (A to select all vertices-->E to extrude-->Z to constrain to Z axis-->1 to extrude one unit-->Enter),
Tab into object mode then add a "solidify" modifier.  Modify the Thickness to your liking - use a positive or negative Thickness to get it to extrude out to the direction you want.
Apply the solidify modifier.
Tab into edit mode and delete the upper/lower layer of vertices, or else move them up/down to the sidewalk thickness you prefer.
Do the same for the other edge of your road.


Should do the trick!  At least it did with a sample curve I played with.  I don't know if it's shaped like your road.

Edit: here is a screenshot of the sample I worked with:



I started with the light gray road mesh (which I actually created from a bezier curve), then I created the two light blue sidewalk objects using the steps described above.